Have you considered making the app to take a GPS reading then turn off the GPS to save battery life? If the person say is trapped in rubble they likely not going to move afterwards thus you can relay this location information to nearby receivers.
This is really smart. I did not!
In my situation, GPS was being jammed. That made GPS not useful at all. I tried my best to avoid it for that and battery usage.
But this initial reading and turning it off is just genius. I added to the list. Thank you so much!